As an example of a display device, a transmission type liquid crystal display device that performs display using light emitted from an illumination device is known. This type of liquid crystal display device has a liquid crystal panel, and an illumination device arranged on a back side of the liquid crystal panel. An illumination device of the related art includes a light source such as a light emitting diode (hereinafter referred to as an LED), and a light guide plate. In the illumination device of the related art, generally, light emitted from the light source is propagated inside the light guide plate and emitted uniformly from an entire surface of the light guide plate.
Hereinafter, in this specification, the illumination device provided on the back side of the display panel as described above may be referred to as a backlight.
On the other hand, an illumination device that emits light selectively from a specific region within a surface of a light guide plate has been developed. In a liquid crystal display device including this type of illumination device, for example, there are a region that displays black on a liquid crystal panel, and a region that displays other colors. Whether the light from the illumination device is to be emitted or not is controlled in each region so that the light is not emitted from the illumination device in the region that displays black on a liquid crystal panel and the light is emitted from the illumination device in the region that displays other colors. If the illumination device performs such control, a phenomenon in which a black display portion appears whitish, i.e., a so-called “black floating” phenomenon, is suppressed, thus improving contrast of the display. Further, a function of adjusting an amount of emitted light from each region, i.e., a so-called light-controlling function, as well as controlling lighting/non-lighting in each individual region may be added to the illumination device. In this case, it is possible to expand a contrast range for representation and produce a powerful image by light-controlling the illumination device according to an image displayed by the liquid crystal panel.
For example, as an example of a method for controlling illumination light, a display device having a configuration in which a light control panel having a light control layer, such as polymer dispersed liquid crystal, is closely adhered to a lower surface of a light guide plate that guides the illumination light from a light source is disclosed (refer to Patent Reference 1 noted below). The light control panel provided in this display device has a configuration in which the polymer dispersed liquid crystal is interposed between a light-transmitting glass substrate and another substrate. The light-transmitting glass substrate has a transparent electrode formed on its entire surface. In the other substrate, a lattice-shaped electrode is formed. Also, a voltage is applied to the polymer-dispersed liquid crystal using the transparent electrode and the lattice-shaped electrode to electrically change a light scattering degree of the polymer dispersed liquid crystal, thereby controlling extraction of light from a light guide plate.
Further, as another example of a method of controlling the illumination light, a liquid crystal display device including a liquid crystal panel, a light source for irradiating the illumination light to the liquid crystal panel, a complex type light guide plate, and a polymer-dispersed liquid crystal plate is disclosed (refer to Patent Document 2 noted below). The complex type light guide plate includes a plurality of divided light guide plates provided below the liquid crystal panel. The polymer-dispersed liquid crystal plate includes a plurality of divided areas on which the light from the light source is incident. In this liquid crystal display device, the illumination light from the light source is incident on the complex type light guide plate through the polymer-dispersed liquid crystal plate. In this case, a distribution of a gradation level of a display pixel of the liquid crystal panel in each divided region of the complex type light guide plate is examined, and a judgment result indicating that the distribution is dark or bright as a whole is obtained. Also, light transmittance of the divided area of the polymer dispersed liquid crystal plate corresponding to the divided region of the complex type light guide plate for which the determination result has been obtained is controlled according to the above determination result to perform control of the illumination light.